1. Field of the Invention
The present invention relates to a method of manufacturing an information recording medium in which a recording film is formed on a substrate. More particularly, the present invention relates to a method suitable for manufacturing a magneto-optical recording medium of the domain wall shift type.
2. Description of the Related Art
Various types of optical and magnetic recording mediums are known wherein a recording film is formed on a substrate. An optical recording medium for recording information corresponding to changes in the optical state, which can be detected using light, can record information with a high density and has been practiced in the form of an optical disk, card or the like.
Changes in the optical state of an optical recording medium are provided by pits formed in the medium surface, the presence/absence or deformation of a reflecting film, changes in the refractive index, inversion of magnetization, etc. Of these examples, inversion of magnetization is detected as a change in the state of polarization of, e.g., reflected light, whereas a change in the amount of, e.g., reflected light is detected in the other examples.
Information is recorded on or is reproduced from an optical recording medium forming marks that cause changes in the optical state as a mark train corresponding to the information, and the mark train is time-serially detected as changes in the amount of, e.g., reflected light for reproduction of the recorded information.
In a recording mode, where such changes in the optical state are caused by heating upon irradiation of a laser beam, the size and shape of state-changed areas (recording marks) depend upon a temperature distribution induced on a recording film by local heating. The temperature distribution is affected by diffusion of heat in a direction along the film surface. Therefore, even when a heated area is moved at a constant linear speed, by keeping constant the amount of heat applied per unit time, the temperature distribution formed by the applied heat changes depending upon differences in the distance that the heated area traversed and the distance from the area that has just been heated. For that reason, if the heating is performed simply corresponding to information, the width of the marks formed in accordance with an information pattern varies. Then, if a mark train formed in such a manner is time-serially read by a light beam scan, there is a risk that the original information might be incorrectly reproduced because of an increased timing jitter in detection of the marks. To avoid such a problem, a variety of recording compensation methods have been proposed that record information after adjusting the amount of heat applied per unit time, the heating time, etc. in accordance with an information pattern. Those methods, however, lead to a problem in that a recording device becomes complicated.
Also, narrowing the intervals between information tracks to achieve a higher density results in a problem of cross-erase or cross-write. Specifically, when information is recorded on or erased from one track by heating and laser beam irradiation, information bits stored on the adjacent tracks might be affected due to heat diffusion, if the intervals between the tracks are too small.
Further, in Japanese Patent Laid-Open No. 6-290496, the inventor proposed a magneto-optical recording medium and a high-density recording and reproducing method. The magneto-optical recording medium comprises a first magnetic layer, a second magnetic layer, and a third magnetic layer that are formed successively in a multilayered structure. The first magnetic layer has a coercive force of a domain wall smaller than that of the third magnetic layer, and the second magnetic layer has the Curie temperature lower than those of the first and third magnetic layers. With a high-density recording and reproducing method, the domain wall of the first magnetic layer of the magneto-optical recording medium, which is present at the boundary of a recording mark, is shifted with a temperature gradient, and inversion of magnetization caused by the shift of the domain wall is detected as a polarization change of the reflected light. To stabilize the shift of the domain wall and improve information reproduction, it is desired, in the proposed method, that the domain wall at the front boundary of a recording mark and the domain wall at the rear boundary thereof be independently and separately formed.
In trying to manufacture the proposed medium by forming a recording film on a substrate using a conventional method, however, the formed recording film is substantially uniform in a direction along the film surface, resulting in a difficulty in forming a recording mark that has front and rear walls of a magnetic domain completely separate from each other. Accordingly, the front and rear domain walls have been separated from each other by, for example, annealing both sides of each track with a high-power laser after the formation of the recording film to eliminate or change the magnetic films on both the sides of the track, and forming recording marks in a straddling relation to the annealed areas. However, the necessity of additional annealing has raised a problem in that the process for manufacturing the medium is complicated and the cost is high. It has also been difficult to employ a land/groove recording scheme in which guide grooves formed for servo control on both sides of each information track are also used as information tracks.
To overcome the problems set forth above, it is an object of the present invention to provide a method of manufacturing an information recording medium, which can form a train of marks of constant width regardless of an information pattern and can also reduce an error rate in reproducing original information even when the information is recorded in a heating mode using a simple recording scheme. Another object of the present invention is to provide a method of manufacturing an information recording medium, which can avoid cross-erase/write even when the intervals between information tracks are decreased.
Still another object of the present invention is to improve the high-density recording and reproducing method proposed in the above-cited Japanese Patent Laid-Open No. 6-290496, for example, in that a characteristic of reproducing information is improved without complicating the method of manufacturing an information recording medium, and that a track density is increased through application of the land/groove recording scheme.
According to the present invention, the above objects are achieved by a method comprising the steps of forming a recording film on a surface of a substrate, including slopes which are positioned on both sides of each information track and have surfaces not parallel to surfaces of the information tracks, and sputter-etching the surface of the substrate by accelerated ions impinging against the surfaces of the information tracks substantially vertically, whereby the recording film on each slope is etched away while the recording film remains on the surface of each information track.
In an information recording medium obtained according to the manufacturing method of the present invention, the recording film is hardly present on the slope between the information tracks. Therefore, even when data is recorded on the information track with possible protrusion onto the slopes on both sides of the information track, the recording is essentially not made on the slopes. As a result, even when recording information by a heating process, the width of the marks formed for recording can be restricted to the information track and a mark train having a predetermined width can be formed regardless of any information pattern.
Most heat produced during the recording or erasing operation is conducted through the recording film and then diffused in a direction along the film surface. In the information recording medium according to the present invention, heat conducted through the recording film is remarkably prevented from further diffusion by the presence of the slope between the information tracks, in which the recording film is hardly present. Therefore, a temperature increase of the tracks adjacent to the information track under heating and cross-erase/write can be prevented.
Further, when the present invention is applied to the recording medium proposed in the above-cited Japanese Patent Laid-Open No. 6-290496, the slopes on both the sides of each information track can be formed to exhibit no magnetism to avoid accumulating the energy of magnetic domain walls. Thus, by forming a heated area for recording in a straddling relation to the recording track and the slopes on both the sides thereof, a recording mark can be formed as a magnetic domain in which no domain walls are present at the sides of the magnetic domain and front and rear domain walls on the recording track are essentially separated from each other.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.